Young girls with Fragile X Syndrome, as well as a comparison group of developmentally delayed girls known to test negative for Fragile X, were characterized on the basis of parent-rated behavioral checklists, including the Conners and Autism Checklists. Girls with a diagnosis of Fragile X showed an increase in a number of areas, including hyperactivity and autistic-like behaviors, relative to the comparison group, which persisted after statistical adjustment for age and IQ. Another notable finding was the tremendous variability seen among members of the Fragile X group. Results based upon parent questionnaires paralleled reports based upon professional examinations, suggesting that these instruments may prove useful for screening in a variety of settings.